This invention relates to a slip-controlled brake system for automotive vehicles which has a braking pressure modulator operated as a function of the deceleration and having an inert mass which is coupled to the slip-controlled wheel. The modulator rotates with the wheel, is axially displaceable against a resetting force upon a deceleration of the wheel, and actuates a valve member after a wheel deceleration threshold has been exceeded, thereby bringing about a modulation of the pressure in the brake of the controlled wheel. The speed of the rotating inert mass corresponds to the speed of the vehicle wheel or to a multiple of the speed.
A control circuit for controlling the braking force of vehicles is known, the circuit having a device measuring the rotating deceleration and having a rotating mass (DE-AS=German Printed and Examined Patent Application No. 16 55 394). By way of a coupling device, the rotating mass is connected with the slip-controlled vehicle wheel by way of a shaft. A wheel deceleration threshold being surpassed, the inert mass at first will maintain its speed due to its inertia. This will result in a speed difference between the shaft and the rotating mass, the mass being axially displaced via a mechanism with a ball and an inclined ramp. By way of a mechanical actuating member, said movement will be transmitted to a variable electrical resistor arranged in a branch of a Wheatstone bridge circuit.
A second resistor in the bridge circuit changes its value proportionally to the braking pressure. If there is a change in the two variable resistor ratio this will result in a detuning of the Wheatstone bridge. Thereby a solenoid valve will be actuated which will modulate the braking pressure of the controlled wheel. Such control circuits are relatively complicated and expensive as they require mechanical as well as electric and electromagnetic components.
For the purpose of slip control already there are also known braking pressure modulators of a purely mechanical type. These modulators contain an arrangement for measuring the rotating deceleration which is similar to the previously described device. In this case, however, by way of mechanical coupling (a lever) the axial displacement of the inert mass, when a wheel deceleration threshold is surpassed, directly leads to the actuation of a valve member bringing about a pressure reduction in the associated controlled wheel and thereby counteracting the lock-up tendency manifest in the high deceleration (SAE Technical Paper Series, No. 840464, Newton, Riddy, Intern. Congress Febr. 27--Mar. 2, 1984). A disadvantage of these modulators consists in that a fixed value is predetermined as a deceleration threshold value which has to lie in the order of magnitude of or even exceeds the maximum vehicular deceleration possible of 1 g, ("g" in this case meaning the constant of acceleration due to gravity). Otherwise, in case of a dry non-skid road surface, the braking pressure would be reduced too early, this leading to an excessive lengthening in the stopping distance as compared with an uncontrolled braking action. Thus, instable behavior of the wheel will be recognized only when the wheel deceleration has amounted to a value above the threshold value of, for example, 1.0 to 1.5 g.
In case of winter road conditions or of aquaplaning, however, the maximum vehicular deceleration possible often lies within the range of only 0.1 to 0.3 g, and thus a wheel deceleration of such magnitude indicating an instability or rather imminent wheel lock-up. Thus, control will start relatively late, namely not until the fixed predetermined deceleration threshold of 1 to 1.5 g is surpassed. The wheel slip then will reveal values which will entail less longitudinal and lateral forces as compared with conditions in case of optimum slip.
This relatively high deceleration threshold further has the disadvantage that in case of cautious brake application on slippery road surfaces, possibly even assisted by high mass moments of inertia of the coupled wheels, drive shafts, and gear rotating masses, the fixed threshold value of 1.0 to 1.5 g will not be reached before lock-up of the wheels. Thus, slip control will not come into operation. Under such circumstances, thus, the deceleration threshold is "run under" as it were.
It is thus an object of this invention to overcome these disadvantages and to further develop a brake system of the type referred to above such as to ensure that even under unfavorable circumstances, such as on slippery road surfaces, control will come about in good time and will recognize instability in each and every case. On the other hand, in case of non-skid road surfaces, the maximum wheel deceleration possible is to be attained without the response of slip control so as to ensure a stopping distance as short as possible.